1. Field of the Invention
The present invention relates to an airbag formed by sewing up a base fabric woven with polar fiber, and to a sewing thread used in the sewing of the airbag. An airbag is a component incorporated into an airbag device installed in such vehicles as automobiles for protecting vehicle occupants.
In this specification, the invention will be described as being applied to an airbag device for a steering wheel. However, the invention can of course be applied to other types of airbag such as an airbag for a front passenger's seat, a side impact airbag and so on.
2. Description of Related Art
Conventionally, such an airbag used in an airbag device is produced by sewing up one or more (usually, more than one) base fabrics woven, usually plain-woven into a bag shape with polar fiber such as polyamide fiber exemplified by nylon 66 and/or the like, as shown in JP 2002-283941.
The sewing work of an airbag shown at 10 in FIG. 1 is performed on a high speed sewing machine (frequency of machine rotation 1000 to 4000 rpm (min−1), usually 2000 to 3000 rpm), using a sewing thread 14 which is made up of the same kind of fiber as that used for base fabrics 12A, 12B, for example nylon 66. In the case of an airbag for a steering wheel, two pieces of circular base fabrics 12A, 12B are sewn up at their peripheral regions.
As in the case of sewing other thick fabrics, the sewing thread usually used for the above sewing work on a high speed sewing machine is the thread that has been subjected to a process of lubrication (dipping treatment) with a lubricant (usually, oleum) such as a silicone oil (a silicone-based oleum) from the viewpoint of preventing the thread breaking due to the heat of a needle.
On the other hand, recently, an improvement of air tightness of airbags upon airbag deployment has been increasingly demanded.
However, conventional airbags whose peripheral regions are sewn up with the sewing yarn treated by silicone oil has hardly fulfilled the demand.